Anisotropic etching of silicon gates is a key step in today's integrated circuit fabrication. For sub-100 nm gate dimensions, one of the main issues is to precisely control the shape of the etched feature. This requires a detailed knowledge of the various physicochemical mechanisms involved in anisotropic plasma etching. Since silicon etching in high-density plasmas is strongly ion assisted, the identities of the ions bombarding the wafer is a key parameter that governs the etch rates and the etched profiles. In the present article, mass spectrometry has been used to investigate the chemical composition of the ion flux bombarding the reactor walls of an industrial inductively coupled plasma used for 200-mm-diam silicon wafer processing. The plasma chemistries investigated are HBr/Cl-2/O-2 and HBr/Cl-2/O-2/CF4 mixtures optimized for sub-100 nm gate processes. Quantitative ion mass spectra show that under those conditions the ion flux contains up to 50% of SiClxBry+ (X, Y= 0-2) ions, although Cl+, Cl-2(+), and Br+ ions were expected to be the predominant species. This observation can be explained by the combination of two well-accepted phenomena that are discussed in detail. The impact of the surprisingly large amount of ionized silicon-based etch products on silicon etching mechanisms are discussed.